Potentially hazardous atmospheres are found in many locations. These hazardous atmospheres exist due to the presence of toxic gases, combustible gas mixtures or the excess or deficiency of oxygen.
Many types of gas detection instruments have been developed to provide a warning that the atmosphere contains potentially hazardous components, or to initiate remedial action. Examples of these gas detection instruments include the detection of combustible gases in coal mines, hydrogen sulfide in oil fields and water treatment plants, carbon monoxide in places ranging from steel mills to bedrooms, and oxygen in confined spaces, such as sewers.
The reliability of toxic gas detectors is of great importance in many applications, especially when these instruments are used for ensuring the safety of personnel. Reliability is typically obtained by periodic checking of the instrument response to a test gas, however calibration test gases are typically supplied in large, bulky and expensive gas cylinders.
Some existing gas detection instruments include one or more gas sensors, whose function is to provide an electrical signal, which varies in response to the gas concentration. The output from many types of sensors can vary over time and sensors can fail to operate without warning.
Most gas sensors provide a relative output signal, such that the output signal is not an absolute measure of gas concentration, but merely proportional to the gas concentration. In such cases, the gas sensor must be calibrated with a known test gas prior to use.
Calibration may also be required as a function check to ensure the sensor is working. Calibrating a gas sensor can often times be time consuming, expensive and cumbersome in many applications.
Some conventional sensors may suffer from unpredictable baseline drift and span drift. If the sensor is not sensitive or fast enough, unacceptable undetected toxic analyte events or high False Alarm Rates (FAR) may occur.
Photoacoustic sensors may be used to detect sample gases based on the tendency of molecules of sample gases to reach higher relative levels of molecular vibration and rotation when the sample gases are exposed to certain frequencies of radiant energy. These higher relative levels of molecular vibration and rotation cause the sample gases to reach a higher temperature and pressure.
When the amplitude of the radiant energy is modulated, the resulting fluctuations in energy available for absorption produce corresponding temperature and pressure fluctuations. A sensitive detector can be used to generate electrical output signals that represent the pressure fluctuations of the sample gases.
These corresponding electrical output signals can be analyzed to evaluate properties or attributes of the sample gases. However, conventional photoacoustic sensors have a sensitivity to environmental acoustic noise sources, especially when the environmental acoustic noise sources exist at the sensors operating frequency.
One of the drawbacks with existing photoacoustic sensors is that since each environment has a unique noise content, most existing photoacoustic sensors are not typically able to readily separate the acoustic noise from the desired relevant photo acoustic signal by filtering. Therefore, a need exists for a photoacoustic sensor that minimizes acoustic noise interference when analyzing electrical output signals to evaluate properties or attributes of sample gases.